One type of processing apparatus for semiconductor wafers is a single wafer processor in which one wafer at a time is supported on a susceptor in a processing chamber. The susceptor divides the chamber into one portion which is below the susceptor (the lower portion), and a second portion which is above the susceptor (the upper portion). The susceptor is generally mounted on a shaft which rotates the susceptor about its center to achieve a more uniform processing of the wafer. A flow of a processing gas, such as a deposition gas, is provided in the upper portion of the chamber and across the surface of the wafer. The chamber generally has a gas inlet port at one side thereof, and a gas outlet port at an opposite side to achieve a flow of the processing gas across the wafer. The susceptor is heated in order to heat the wafer to a desired processing temperature. One method used to heat the susceptor is by the use of lamps provided around the chamber and directing their light into the chamber and onto the susceptor. In order to control the temperature to which the wafer is being heated, the temperature of the susceptor is constantly measured. This is often achieved by means of an infrared temperature sensor which detects the infra-red radiation emitted from the heated susceptor.
With this type of processing apparatus, some of the processing gas, which is often a gas or mixture of gases for depositing a layer of a material on the surface of the wafer, tends to flow around the edge of the susceptor and deposits a layer of the material on the back surface of the susceptor. Since the deposited material is generally different from the material of the susceptor, the deposited layer has an emissivity which is different from that of the emissivity of the susceptor. Thus, once the layer of the material is deposited on the back surface of the susceptor, the infrared temperature sensor detects a change caused by the change in the emissivity of the surface from which the infra-red radiation is emitted. This change indicates a change in temperature of the susceptor which actually does not exist.
There is a need, therefore, to provide apparatus and methods which ensure constant emissivity of the backside of susceptors. There is a continuing need to achieve uniform temperature of susceptors, and thereby achieve uniform selective epitaxial- and non-epitaxial (e.g., poly-silicon) deposition.